1. Field of the Invention
The invention comprises a method for managing power flow in a hybrid electric vehicle powertrain with a primary power source and an electric motor power source wherein primary power source vibrations during starting and stopping are attenuated.
2. Background Discussion
In a hybrid electric vehicle powertrain such as that disclosed in co-pending U.S. patent application Ser. No. 10/605,313, filed Sep. 22, 2003, entitled “A Controller and Control Method for a Hybrid Electric Vehicle Powertrain,” which is assigned to the assignee of the present invention, an electric motor and an engine are used to distribute power through transmission gearing to vehicle traction wheels. The gearing establishes plural power flow paths from the engine to vehicle traction wheels, a reaction element of the gearing being drivably connected to a generator in a battery-motor-generator electric power source configuration. The generator and the engine in this configuration are mechanically coupled through the gearing. The battery acts as an energy storing medium for the generator and the motor.
In one operating mode, the engine, which acts as a primary power source, will develop driving torque in a forward direction as the generator establishes a reaction torque. In the alternative, reaction torque can be established by a generator brake during fully mechanical engine drive.
Because of the characteristics of the planetary gearing in a powertrain configuration of this type, the engine speed can be considered to be decoupled from the vehicle speed, the engine speed being determined by a generator speed control. This results in divided power delivery paths, a first being a mechanical path from an engine torque output element and a second being an electrical path in which electrical power is distributed to the motor to which the generator is electrically coupled. Because of the decoupling of the functions of the electrical power flow path and the mechanical power flow path, the transmission consisting of the motor, the generator and the gearing, can be regarded as having power delivery characteristics similar to that of a conventional continuously variable transmission (CVT).
In a powertrain configuration of this type, in which the motor is a secondary power source, the electric motor draws power from the battery and provides propulsion independently of the engine, thus driving the vehicle in either a forward direction or a reverse direction. Further, the electric motor can provide electrical braking torque and recover vehicle kinetic energy during vehicle braking. That kinetic energy, in a conventional powertrain without hybrid characteristics, would otherwise be lost in the form of heat. The recovered vehicle kinetic energy of a hybrid electric vehicle powertrain can be used to charge the battery. Further, the generator can act as a motor using battery power while a one-way coupling for the gearing serves as a reaction member, whereby the vehicle can be propelled in a forward direction.
The primary power source and the secondary power source can simultaneously propel the vehicle in a forward direction to meet the driver's demand for torque and to achieve better acceleration performance.
Conventional CVT powertrains for vehicles make it possible to achieve better fuel economy and to reduce undesirable engine exhaust gas emissions by operating the engine in its most efficient speed and torque operating region whenever possible. Hybrid powertrains of the kind discussed in the preceding paragraphs have a potential for improving fuel economy and for reducing undesirable exhaust gas emissions even more effectively compared to conventional CVT powertrains equipped vehicles. This is due to the fact that the engine size can be reduced while providing the same vehicle performance resulting from the use of two power sources. It is due also to the fact that engine operation can be better optimized since the engine can be stopped if the required engine operating conditions are not favorable for high fuel economy and reduced undesirable exhaust gas emissions. Further, as previously mentioned, the regenerative kinetic energy developed during vehicle engine braking can be captured and stored in the battery.
In order to integrate the dual power sources to work together seamlessly to achieve improved performance, fuel economy and reduced undesirable engine exhaust gas emissions, coordination of the control of the power sources is needed. This control is achieved, as disclosed in the co-pending patent application identified above, by using a hierarchal vehicle system controller to control and manage power distribution from each of the two power sources. Under normal powertrain operating conditions, when the powertrain sub-systems and components are functional, the vehicle system controller will interpret a driver demand for acceleration or deceleration and then determine the traction wheel torque command based on the driver demand within predetermined powertrain power limits, including battery power limits. Further, the vehicle system controller determines when and how much power each power source needs to provide in order to meet the driver's demand and to achieve the specified vehicle performance; i.e., fuel economy, reduced undesirable engine exhaust gas emissions, drivability, etc. Thus, the vehicle system controller will determine when the engine must be turned off and when it must be turned on. It determines also the engine speed and engine torque operating point for any given power demand when the engine is operating.
One of the measures that can be taken to achieve better fuel economy and to reduce undesirable engine exhaust gas emissions in a hybrid electric vehicle powertrain, as previously explained, is to shut off the engine when the engine cannot be operated in its desired efficient operating region; for example, when the vehicle is stopped at a traffic light during vehicle operation in urban regions. As a result, the engine in the hybrid electric vehicle powertrain, unlike a conventional powertrain in which the engine must be started in a first start-up of the vehicle and shut off only by an ignition key, will be started and stopped relatively frequently during normal urban driving. These hybrid electric vehicle engine start and stop events can be unexpected to a driver. Thus, they are required to be imperceptible.
It is desirable for an engine start to be free of oscillatory seat-track acceleration. In reality, an engine start will tend to cause undesirable vibrations characterized by harshness. Such vibrations and harsh powertrain operating events may occur in two separate phases, where they are perceptible to a driver. The first phase is an engine speed ramp-up or cranking phase. The second is an initial engine combustion phase. These two separate phases are caused by two vibration sources, the first being the compression forces in the engine cylinders during engine speed ramp-up and the second being abrupt initial combustion forces during an engine start.
During the engine cranking phase, the generator in a powertrain of the type disclosed in the co-pending application provides a cranking torque to ramp-up the engine to achieve engine start or engine ignition, which results in cylinder compression forces. These compression forces and inertia forces caused by reciprocating motion of engine pistons can excite the engine's natural torsional vibration mode. If the engine speed is not controlled properly, natural torsional vibration frequencies are noticeable to the vehicle driver as the vibrations pass through an engine torsional resonance range. This can cause engine block “shaking” as well. Further, the generator cranking torque, when the engine is being cranked by the generator acting as a motor, is transmitted through the driveline because of the mechanical connection between the generator and the vehicle traction wheels. This cranking torque can excite the driveline natural torsional vibration mode. Similarly, in the initial combustion phase of an engine start-up event, the abrupt initial engine combustion torque can excite the engine's torsional vibration mode as well as the driveline torsional vibration mode.
The cylinder compression forces can be experienced also during engine ramp-down as the engine is shut off by the vehicle system controller.
The resonant vibrations of the engine and the driveline and the “shaking” of the engine result in vehicle body vibrations and harshness transmitted through powertrain mounts in the vehicle chassis.
U.S. Pat. No. 6,247,437 discloses a hybrid electric vehicle powertrain with an engine and two motors that cooperate with a planetary torque splitter gear unit. The controller used in managing power flow from the engine and the motors defines a generator torque command profile for starting the engine.
As in the case of the powertrain of the co-pending patent application, the generator of the powertrain of the '437 patent is used to provide engine starting torque. The torque profile varies based upon whether the engine is cold. The controller is an open loop control for managing engine speed as a driver torque command is applied for engine starting. There is no feedback closed-loop feature to compensate for uncertain operating conditions that can result in undesired engine start-up vibrations. Further, there is a subsequent transition to a speed feedback closed-loop control after the engine is started. This transition from an open-loop control to a closed-loop feedback control, in itself, can introduce undesirable engine vibrations and harshness during the initial combustion period.
Another example of a hybrid electric vehicle powertrain using an engine, two motors and a planetary gear unit for establishing plural torque flow paths from the motors and the engine to vehicle traction wheels is disclosed in U.S. Pat. No. 6,278,195. This patent describes a generator torque command profile to ramp-down engine speed during engine shut-off in a manner similar to the open-loop control of the '437 patent. The '195 patent does not deal with the possibility that the engine could be subjected to an instantaneous reverse driving torque during engine stops. There is no compensation for a torque reversal that can take place because of engine vibrations. Furthermore, there is no active damping in either the '195 patent or the '437 patent to suppress driveline oscillations during engine stops and starts.